1. Field of the Invention
The present invention relates to curable hot melt compositions including silicone urea based polymers, which behave like a thermoplastic polymer until crosslinking transforms them into a thermoset. The silicone urea based polymers are reinforced by organic polymers, which allow tailoring of the physical properties of the composition, including hardness, tensile strength and elongation.
2. Brief Description of Related Technology
Thermoplastic polymers are a well-known and widely used class of polymers. Thermoplastic polymers allow for trial and error in attempts to form them into desired shapes. If the thermoplastic polymer is incorrectly molded, it can be re-heated and set again. However, thermoplastic polymers have the disadvantage that a thermoplastic polymer set in an intended form may lose its shape when subjected to temperatures sufficient to cause softening or flow. This can be disadvantageous when the thermoplastic polymer has been set in a desired shape.
Among known thermoplastic polymers are hot melt adhesives. These polymers remain solid until they are heated to the appropriate temperature to make them flow. Hot melts typically form adhesive bonds with the substrate to which they are applied once the flowable polymer is cooled again to a solid.
Contrary to thermoplastic polymers, thermoset polymers avoid the difficulties associated with thermoplastic polymers in that once they are crosslinked, they retain their shape. Unfortunately, this ability that allows the thermoset polymer to maintain its shape can also be a disadvantage—once the thermoset polymer is set in a particular form, it cannot be changed, even if that particular form is not the intended or desired form. Thus, the thermoset polymer must be set in its correct form the first time it is set.
Rheology control of such compositions has also been a concern. For example, compositions formulated to have a high melting range frequently also have a high viscosity, which can make dispensing of the hot melt difficult. Likewise, the use of a higher molecular weight siloxane in preparation of hot melt compositions can lead to a lower content of hard segments, which can result in poor mechanical properties.
Moreover, hot melt adhesives typically exhibit low tensile strength and elongation, which may be undesirable in some applications.
Accordingly, there is a need for a crosslinkable composition that behaves like a hot melt initially, but when exposed to conditions of cure, transforms into a crosslinked composition. There further is a need for such cured compositions to exhibit high tensile strength and elongation properties. Such compositions should be able to maintain a desired shape without fear of loss of physical properties and structural integrity due to temperature fluctuations.